In order to change the default styles you should override the corresponding CSS selector e.g as shown below:
Note, that you should use the keyword !important
in order to provide a higher priority to your custom style.
You can learn more about changing default styles of RadControls in the article below:
As to your second question, the easiest way to make the tab look as a horizontal one is to set bigger width to the sliding zone, set TabView="ImageOnly"
and put a horizontal image.
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